1. Field of the Invention
The present invention relates to a reflection type optical apparatus for directing light to an illuminated plane through an objective lens located on the optical axis of an image formation lens and for forming an image of the illuminated plane on an image formation plane. The image is formed by reflecting light from the illuminated plane through the objective lens and the image formation lens.
2. Description of the Prior Art
The illuminated plane of such an apparatus must be illuminated as brightly as possible. To this end, a light source having high luminance is required. Further, the illuminated plane must be illuminated as uniformly as possible. Further, the illuminated plane must be illuminated at an appropriate illumination angle (angular aperture).
To satisfy these requirements, a so-called Koehler illumination method, illustrated in FIG. 1, is generally employed. Referring to FIG. 1, numeral 101 denotes an image formation optical system. Numeral 110 denotes an illumination system.
The image formation optical system 101 includes an objective lens 102, an objective lens stop 118, and an image formation lens 103. The objective lens 102, the objective lens stop 118, and the image formation lens 103 are located on an image formation optical axis X. The illumination system 110 illuminates an illuminated plane 105a of an object 105 to be inspected. That is, a beam reflected by the illuminated plane 105a forms an image on an image formation plane 106a through the objective lens 102, the objective lens stop 118, and the image formation lens 103. The objective lens stop 118 is located at a rear focal point F.sub.1 of the objective lens 102 to regulate the diameter of the beam reflected by the plane 105a. Thus, the objective lens stop 118 defines the angular aperture 2.theta. of the objective lens 102.
The illumination system 110 includes a light source 111, a condenser lens 112, an aperture stop 113, a field stop 114, a relay lens 115, and a beam splitter 117. The beam splitter 117 is located between the objective lens stop 118 and the image formation lens 103. The aperture stop 113 is conjugate to the objective lens stop 118 through the relay lens 115. The field stop 114 is conjugate to the illuminated plane 105a through the relay lens 115 and the objective lens 102. An illuminating beam B.sub.1 from the light source 111 passes through the condenser lens 112, the aperture stop 113, the field stop 114, and the relay lens 115, and is directed toward the objective lens 102 along the image formation optical axis X by the beam splitter 117. A tertiary image of the light source 111 is formed at an imaginary plane at the point F.sub.1 of the objective lens stop 118 by the condenser lens 112 and the relay lens 115. Therefore, the tertiary image of the light source 111 as viewed from the illuminated plane 105a is located at infinity. Thus, there is no irregularity in the illuminating beam B.sub.1 applied to the illuminated plane 105a and the plane 105a is uniformly illuminated by the beam B.sub.1.
Since the aperture stop 113 and the objective lens stop 118 are conjugate to each other, the size of the tertiary image formed at the point F.sub.1 can be controlled by the aperture stop 113. When the aperture stop 113 is contracted, the angular aperture of the illuminating beam B.sub.1 applied to the illuminated plane 105a is reduced. When the aperture stop 113 is opened, the size of the tertiary image formed at the point F.sub.1 is increased. However, since only a beam having a diameter which is less than or equal to the aperture size of the objective lens stop 118 can pass through the objective lens stop 118, the angular aperture of the illuminating beam B.sub.1 is no greater than the value (2.theta.) corresponding to the aperture size of the lens stop 118 even if the size of the tertiary image exceeds the aperture size of the lens stop 118. In other words, the angular aperture of the illuminating beam B.sub.1 can be controlled within a range of 0.degree. to 2.theta. by adjusting the size of the aperture stop 113. Yet it is impossible to set the angular aperture of the illuminating beam B.sub.1 at a value exceeding 2.theta..
The area illuminated by the beam B.sub.1 can be controlled by adjusting the size of the field stop 114.
As illustrated in FIG. 2, it is not unusual for the illuminated plane 105a to be partially swollen at an inclination angle .theta.. So that such a swell does not reduce the luminous energy directed upon the image formation plane 106a, an angular aperture 2.alpha. of the illuminating beam B.sub.1 and the angular aperture 2.theta. of the objective lens 102 must be adjusted to satisfy the following expression with respect to the inclination angle .phi.: EQU .theta.&lt;.alpha.-.theta. (1)
When the angular apertures 2.alpha. and 2.theta. are adjusted to satisfy the expression (1), the entire region R (corresponding to the angular aperture 2.theta. of the objective lens 102) is included in the optical path of a beam B.sub.2 reflected by the illuminated plane 105a. As a result, the entire reflected beam corresponding to the region R is directed onto the image formation plane 106a.
In the conventional reflection type optical apparatus, however, the angular aperture 2.alpha. of the illuminating beam B.sub.1 cannot exceed the angular aperture 2.theta. of the objective lens 102, for the reasons described above. Therefore, when the illuminated plane 105a is not flat, it is impossible to adjust the angular apertures 2.alpha. and 2.theta. to satisfy the expression (1). As a result, the swell reduces the luminous energy available for forming the image on the plane 106a.
To form an image of the illuminated plane 105a on the plane 106a when the illuminated plane 105a is partially swollen at the inclination angle .theta., the angular apertures 2.alpha. and 2.theta. must satisfy the following expression with respect to the inclination angle .phi.: EQU 2.phi.&lt;.alpha.+.theta. (2)
When this condition is satisfied, the optical path of a reflected beam B".sub.2 reflected by the illuminated plane 105a partially overlaps the region R so that a reflected beam corresponding to the overlap (not illustrated) forms an image on the image formation plane 106a through the image formation optical system 101. In other words, when the angular aperture 2.alpha. of the illuminating beam B.sub.1 and the angular aperture 2.theta. of the objective lens 102 do not satisfy the; expression (2) (as illustrated in FIG. 3), the optical path of the beam B".sub.2 reflected by the illuminated plane 105a will not overlap the region R. As a result, the image of the illuminated plane 105a will not be observable by the reflection type optical apparatus.
Thus, a value (.alpha.+.theta.) must be increased when the illuminated plane 105a is not flat. Since the angular aperture 2.alpha. of the illuminating beam B.sub.1 cannot exceed the angular aperture 2.theta. of the objective lens 102 (for the reasons stated above), the angular aperture 2.theta. must be increased to increase the value (.alpha.+.theta.).
However, depth of focus Q (FIG. 4) is inversely proportional to the square of sin.theta.. Thus, ##EQU1## where .lambda. is the wavelength of the reflected light.
Therefore, it is desired to reduce the angular aperture 2.theta. of the objective lens 102 to increase the depth of focus Q, when a non-planar substance is to be observed by the system 101.
However, the objective lens stop 118 must be contracted to reduce the angular aperture 2.theta. of the objective lens 102, and the angular aperture 2.alpha. of the illuminating beam B.sub.1 is inevitably reduced in response to such contraction of the objective lens stop 118. When the angular aperture 20 is reduced to increase the depth of focus Q, the following problems are caused:
First, illuminance of the illuminated plane 105a is decreased as the angular aperture 2.alpha. of the illuminating beam B.sub.1 is reduced. Second, the expression (2) is not satisfied because of the reduction of the angular aperture 2.theta. of the objective lens 102 and the angular aperture 2.alpha. of the illuminating beam B.sub.1 when the illuminated plane 105a is not flat. As a result, the image of the illuminated plane 105a is not observable.
When the angular aperture 2.theta. of the objective lens 102 is increased to satisfy the expression (2) to observe the image of the non-flat illuminated plane 105a, the depth of focus Q is reduced.